Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+(or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
NaNi0.5Mn0.5O2 is a known Na-ion material in which the nickel is present as Ni2+ while the manganese is present as Mn4+. The material is ordered with the Na and Ni atoms residing in discrete sites within the structure. The nickel ions (Ni2+) are a redox element which contributes to the reversible specific capacity and the manganese ions (Mn4+) play the role of a structure stabilizer. Compound NaNi0.5Ti0.5O2 is analogous to NaNi0.5Mn0.5O2 in that the Ni2+ ions provide the active redox centre and the Ti4+ ions are present for structure stabilization. There is plenty of literature describing the preparation of NaNi0.5Mn0.5O2 (and to a lesser extent NaNi0.5Ti0.5O2) as the precursor for making LiNi0.5Mn0.5O2 and LiNi0.5Ti0.5O2 by Na→Li ion exchange for Li-ion applications. A direct synthesis method to make these Li materials may yield undesirable disordered materials, for example, as a result of the lithium and nickel atoms sharing structural sites. However, recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi0.5Mn0.5O2 electrodes in propylene carbonate electrolyte solutions. The results obtained show that although NaNi0.5Mn0.5O2 exhibits some reversible charging and discharging ability, the capacity of the material fades by 25% or more, after only 40 cycles.
Work is now being undertaken to find even more efficient electrochemically active materials, which have large charge capacity, are capable of good cycling performance, highly stable, and of low toxicity and high purity. Of course, to be commercially successful, the cathode materials must also be easily and affordably produced. This long list of requirements is difficult to fulfil but it is understood from the literature that the active materials which are most likely to succeed are those with small particle size and narrow size distribution, with an optimum degree of crystallinity, a high specific surface area, and with uniform morphology.
The present Applicant has also now conducted work which demonstrates that electrochemical activity is further optimised when the active material includes metal constituents with certain defined oxidation states. Furthermore the Applicant has identified active materials with a specific crystal structure to be especially active.
The present invention aims to provide novel compounds. Further the present invention aims to provide a cost effective electrode that contains an active material that is straightforward to manufacture and easy to handle and store. Another aim of the present invention is to provide an electrode that has a high initial specific discharge capacity and which is capable of being recharged multiple times without significant loss in charge capacity.
Therefore, the first aspect of the present invention provides compounds of the formula:A1-δM1VM2WM3XM4YM5ZO2                 wherein        A is one or more alkali metals comprising sodium and/or potassium, either alone or in a mixture with lithium as a minor constituent;        M1 is nickel in oxidation state +2        M2 comprises a metal in oxidation state +4 selected from one or more of manganese, titanium and zirconium;        M3 comprises a metal in oxidation state +2, selected from one or more of magnesium, calcium, copper, zinc and cobalt;        M4 comprises a metal in oxidation state +4, selected from one or more of titanium, manganese and zirconium;        M5 comprises a metal in oxidation state +3, selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium;        wherein        0≦δ≦0.1        V is in the range 0<V<0.5;        W is in the range 0<W≦0.5;        X is in the range 0≦X<0.5;        Y is in the range 0≦Y<0.5;        Z is ≧0;        and further wherein V+W+X+Y+Z=1.        
Preferably the present invention provides a compound of the above formula wherein V is in the range 0.1≦V≦0.45; w is in the range 0<W≦0.5; x is in the range 0≦X<0.5; Y is in the range 0≦Y<0.5; Z is ≧0; and wherein V+W+X+Y+Z=1.
Further preferably the present invention provides a compound of the above formula wherein V is in the range 0.3≦V 0.45; W is in the range 0.1≦W≦0.5; X is in the range 0.05≦X<0.45; Y is in the range 0≦Y≦0.45; Z is ≧0; and wherein V+W+X+Y+Z=1.
In particularly preferred compounds of the above formula, V is in the range 0.3≦V<0.45; W is in the range 0<W≦0.5; X is in the range 0≦X≦0.3; Y is in the range 0≦Y≦0.4; and Z is in the range 0≦Z≦0.5.
Compounds of the above formula in which δ=0.05, are highly beneficial.
In additionally preferred compounds of the present invention M2≠M4.
It is particularly advantageous if V+W+Y<0.9 in the compounds of the present invention.
The present Applicant has found that not only are the oxidation states of the metal constituents in the compounds of the present invention a critical feature to the production of highly electrochemically active compounds but they have also confirmed that having metal constituents with these particular oxidation states will determine the overall crystalline structure of the compound. It is known that that there are several possible layered structural forms which alkali metal/metal/oxides may adopt, including O3, P3 and P2. The Applicant has shown that the oxidation states for the metal constituents cause a particular structure to be adopted and in particular has determined that alkali metal/metal/oxide compounds with a metal in +4 oxidation state and with a sodium content close to 1, will adopt an O3 crystalline structure. Moreover, the Applicant has demonstrated that alkali metal/metal/oxides with the metal in oxidation state +4 and with an O3 crystalline structure exhibit a much higher electrochemical activity than similar compounds that do not contain a metal in +4 oxidation state. The Applicant has also observed that the materials without a metal in +4 oxidation state, typically have a P2 crystalline structure, thus there appears to be a very strong correlation between crystalline structure, oxidation state and electrochemical activity.
Hence, the present invention provides preferred compounds of the formula:A1-δM1VM2WM3XM4YM5ZO2 in an O3 layered structural formwherein                A is one or more alkali metals comprising sodium and/or potassium either alone or in a mixture with lithium as a minor constituent;        M1 is nickel        M2 comprises a metal selected from one or more of manganese, titanium and zirconium;        M3 comprises a metal selected from one or more of magnesium, calcium, copper, zinc and cobalt;        M4 comprises a metal selected from one or more of titanium, manganese and zirconium;        M5 comprises a metal selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.        
Especially preferred compounds of the present invention include:    NaNi0.5−x/2Ti0.5−x/2AlxO2;    NaNi0.5−x/2Mn0.5−x/2AlxO2;    NaNi0.5−xMn0.5−xMgxTixO2;    NaNi0.5−xMn0.5−xMgx/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCaxTixO2;    NaNi0.5−xMn0.5−xCoxTixO2;    NaNi0.5−xMn0.5−xCuxTixO2;    NaNi0.5−xMn0.5−xZnxTixO2;    NaNi0.5−xMn0.5−xMgxZrxO2;    NaNi0.5−xMn0.25−x/2CaxTi0.25+x/2O2;    NaNi0.5−xMn0.5CaxO2;    NaNi0.5−xMn0.5−YCaxTiYO2;    NaNi0.5−xTi0.5−xMgxMnxO2;    NaNi0.5−xTi0.5−xCaxMnxO2;    NaNi0.5−xTi0.5−xCuxMnxO2;    NaNi0.5−xTi0.5−xCoxMnxO2;    NaNi0.5−xTi0.5−xZnxMnxO2;    NaNi0.5−xMn0.5MgxO2;    NaNi0.5−xMn0.5CaxO2;    NaNi0.5−xMn0.5CuxO2;    NaNi0.5−xMn0.5CoxO2;    NaNi0.5−xMn0.5ZnxO2;    NaNi0.5−xMn0.5−yMgxTiyO2;    NaNi0.5−xMn0.5−yCaxTiyO2;    NaNi0.5−xMn0.5−yCuxTiyO2;    NaNi0.5−xMn0.5−yCoxTiyO2;    NaNi0.5−xMn0.5−yZnxTiyO2;    NaNi0.5−xMn0.25−x/2Mn0.25−x/2O2;    NaNi0.5−xMn0.25−x/2CaxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2CuxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2CoxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2ZnxTi0.25+x/2O2;    NaNi0.5−xMn0.5−xMgX/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCax/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCux/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCox/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xZnx/2Tix/2AlxO2; and    Na0.95Ni0.3167Ti0.3167Mg0.1583Mn0.2083O2 
Further, extremely preferred compounds of the present invention include:    NaNi0.45Mn0.45Mg0.05Ti0.05O2;    NaNi0.45Mn0.45Ca0.05Ti0.05O2;    NaNi0.45Mn0.45Cu0.05Ti0.05O2;    NaNi0.45Mn0.45Zn0.05Ti0.05O2 and    Na0.95Ni0.3167Ti0.3167Mg0.1583Mn0.2083O2 
In a second aspect, the present invention provides an electrode comprising an active compound of the formula:A1-δM1VM2WM3XM4YM5ZO2                 wherein        A is one or more alkali metals comprising sodium and/or potassium either alone or in a mixture with lithium as a minor constituent;        M1 is nickel in oxidation state +2        M2 comprises a metal in oxidation state +4 selected from one or more of manganese, titanium and zirconium;        M3 comprises a metal in oxidation state +2, selected from one or more of magnesium, calcium, copper, zinc and cobalt;        M4 comprises a metal in oxidation state +4, selected from one or more of titanium, manganese and zirconium;        M5 comprises a metal in oxidation state +3, selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium;        wherein        0≦δ≦0.1        V is in the range 0<V<0.5;        W is in the range 0<W≦0.5;        X is in the range 0≦X<0.5;        Y is in the range 0≦Y<0.5;        Z is ≧0;        and further wherein V+W+X+Y+Z=1.        
Preferably the electrode of the present invention comprises an active compound of the above formula, wherein V is in the range 0.1≦V≦0.45; w is in the range 0<W≦0.5; x is in the range 0≦X<0.5; Y is in the range 0≦Y<0.5; Z is ≧0; and wherein V+W+X+Y+Z=1.
Further preferably the electrode of the present invention comprises an active compound of the above formula, wherein V is in the range 0.3≦V≦0.45; W is in the range 0.1≦W≦0.5; X is in the range 0.05≦X<0.45; Y is in the range 0≦Y≦0.45; Z is ≧0; and wherein V+W+X+Y+Z=1.
Particularly preferred electrodes of the present invention comprise an active compound of the above formula, wherein V is in the range 0.3≦V<0.45; W is in the range 0<W≦0.5; X is in the range 0≦X≦0.3; Y is in the range 0≦Y≦0.4; and Z is in the range 0≦Z≦0.5.
The Applicant has observed that if NiO is present as an impurity phase in samples of the active compounds, then this has a detrimental effect on the electrochemical performance. NiO may be formed during the process of charging the electrode; at this time Ni2+ can be oxidized, using up energy that would normally be used to charge the active material. This is not only an irreversible reaction, but also has a detrimental effect on the cycling performance, resulting in a drop in capacity upon electrochemical cycling. The formation of NiO by this route is found to be minimised by reducing the amount of alkali metal in the active compound and is the purpose for compounds of the invention which have less than 1 unit of alkali metal. However, it is important to maintain sufficient alkali metal in the compound to ensure that it adopts a favourable crystalline structure such as an O3 type structure.
Electrodes comprising active compounds of the above formula in which δ=0.05, are highly beneficial.
Additionally preferred electrodes of the present invention comprise an active compound as described above wherein M2≠M4.
Further preferred electrodes of the present invention comprise compounds of the formula:A1-δM1VM2WM3XM4YM5ZO2 in an O3 layered structural formwherein                A is one or more alkali metals comprising sodium and/or potassium either alone or in a mixture with lithium as a minor constituent;        M1 is nickel        M2 comprises a metal selected from one or more of manganese, titanium and zirconium;        M3 comprises a metal selected from one or more of magnesium, calcium, copper, zinc and cobalt;        M4 comprises a metal selected from one or more of titanium, manganese and zirconium;        M5 comprises a metal selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.        
Especially preferred electrodes of the present invention comprise active compounds selected from one or more of:    NaNi0.5−x/2Ti0.5−x/2AlxO2;    NaNi0.5−x/2Mn0.5−x/2AlxO2;    NaNi0.5−xMn0.5−xMgxTixO2;    NaNi0.5−xMn0.5−xMgx/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCaxTixO2;    NaNi0.5−xMn0.5−xCoxTixO2;    NaNi0.5−xMn0.5−xCuxTixO2;    NaNi0.5−xMn0.5−xMnxTixO2;    NaNi0.5−xMn0.5−xMgxZrxO2;    NaNi0.5−xMn0.25−x/2CaxTi0.25+x/2O2;    NaNi0.5−xMn0.5CaxO2;    NaNi0.5−xMn0.5−yCaxTiYO2;    NaNi0.5−xTi0.5−xMgxMnxO2;    NaNi0.5−xTi0.5−xCaxMnxO2;    NaNi0.5−xTi0.5−xCuxMnxO2;    NaNi0.5−xTi0.5−xCoxMnxO2;    NaNi0.5−xTi0.5−xZnxMnxO2;    NaNi0.5−xMn0.5MgxO2;    NaNi0.5−xMn0.5CaxO2;    NaNi0.5−xMn0.5CuxO2;    NaNi0.5−xMn0.5COxO2;    NaNi0.5−xMn0.5ZnxO2;    NaNi0.5−xMn0.5−yMgxTiyO2;    NaNi0.5−xMn0.5−yCaxTiyO2;    NaNi0.5−xMn0.5−yCuxTiyO2;    NaNi0.5−xMn0.5−yCoxTiyO2;    NaNi0.5−xMn0.5−yZnxTiyO2;    NaNi0.5−xMn0.25−x/2MgxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2CaxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2CuxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2CoxTi0.25+x/2O2;    NaNi0.5−xMn0.25−x/2ZnxTi0.25+x/2O2;    NaNi0.5−xMn0.5−xMgx/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCax/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCux/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xCox/2Tix/2AlxO2;    NaNi0.5−xMn0.5−xZnx/2Tix/2AlxO2; and    Na0.95Ni0.3167Ti0.3167Mg0.1583Mn0.2083O2.
Extremely preferred electrodes comprise active compounds selected from one or more of:    NaNi0.45Mn0.45Mg0.05Ti0.05O2;    NaNi0.45Mn0.45Ca0.05Ti0.05O2;    NaNi0.45Mn0.45Cu0.05Ti0.05O2;    NaNi0.45Mn0.45Zn0.05Ti0.05O2, and    Na0.95Ni0.3167Ti0.3167Mg0.1583Mn0.2083O2.
The electrodes according to the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices.
Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.
In a third aspect, the present invention provides an energy storage device that utilises an electrode comprising the active materials described above, and particularly an energy storage device for use as one or more of the following: a sodium and/or potassium ion cell; a sodium and/or potassium metal cell; a non-aqueous electrolyte sodium and/or potassium ion; an aqueous electrolyte sodium and/or potassium ion cell. In each case lithium may also be present as a minor constituent.
The novel compounds of the present invention may be prepared using any known and/or convenient method. For example, the precursor materials may be heated in a furnace so as to facilitate a solid state reaction process.
A fourth aspect of the present invention provides a particularly advantageous method for the preparation of the compounds described above comprising the steps of:
a) mixing the starting materials together, preferably intimately mixing the starting materials together and further preferably pressing the mixed starting materials into a pellet;
b) heating the mixed starting materials in a furnace at a temperature of between 400° C. and 1500° C., preferably a temperature of between 500° C. and 1200° C., for between 2 and 20 hours; and
c) allowing the reaction product to cool.
Preferably the reaction is conducted under an atmosphere of ambient air, and alternatively under an inert gas.